This invention relates generally to x-ray methods and sources, and more particularly to x-ray methods and sources for the industrial inspection of objects, such as, thick or superalloy parts and the like.
Although x-ray radiography techniques such as computerized tomography (C.T.) and digital fluoroscopy (D.F.) have found wide applicability and have been shown to have significant advantages in some fields, such as medicine, the industrial application of such techniques in certain areas has been hampered by the lack of suitable x-ray sources. One such area is the x-ray inspection of superalloy turbine blades for high performance aircraft engines. Superalloys include nickel, cobalt and iron based alloys which have high strength at high temperatures. The applicability of x-ray radiography techniques to the inspection of such parts has been limited by the low intensity and poor image quality obtainable with conventional x-ray tubes due to the absorption of photons in the material being inspected, as well as by the rather slow speed and lack of resolution and penetration capability of such sources. One way to improve speed, resolution and penetration capability is to increase the intensity of the source. To inspect high atomic number parts, as of superalloys, this implies the need for a high intensity x-ray source capable of operating at substantially higher voltages than currently available sources, preferably of the order of 400-500 kV, and at high power ratings, preferably in the range of tens to hundreds of kilowatts. Moreover, since it is necessary to resolve very small microflaws having a size of the order of thousandths to tens of thousandths of an inch, it is necessary for the source to have a small focal spot size of the order of 1-10 mils in order to obtain high brightness, i.e., intensity, while minimizing power supply requirements.
There are no x-ray sources currently available which satisfy these requirements. Conventional fixed anode x-ray tubes have limited power dissipation capability. Conventional rotating anode x-ray tubes can dissipate substantial amounts of power, but they operate at about 120 kV which are substantially lower voltages than required, and at full power they typically have an electron beam spot size of the order of 1-1.5 mm. To image a 10 mil flaw using such a tube and a detector aperture of the order of 10 mils, it is necessary to operate with the tube approximately nineteen inches away from the detector in order to resolve the flaw. Employing a smaller spot size would enable the source to be moved closer to the part thereby affording advantages in increased resolution and brightness for the same input power to the tube, or alternatively a reduction in power supply requirements for the same brightness. However, a smaller spot size increases the power density incident upon the anode (assuming the input power to the tube remains the same) since the electron beam is focused onto a smaller area of the anode, and increases the temperature rise of the portion of the anode under the spot, which increases the amount of heat which must be transferred from the anode. The maximum input power to an x-ray tube is limited by melting of the anode, and the power rating of conventional tubes is determined by the anode volume in which heat must be dissipated, which is determined by the area of the spot and the depth of diffusion of the heat. Accordingly, it has not been thought possible to realize a high voltage, high power microfocus x-ray tube having the characteristics required for optimum inspection of superalloy parts.